There is studied application of a nitride semiconductor, utilizing characteristics such as a high saturation electron velocity and a wide band gap, to a semiconductor device of a high withstand voltage and a high power. For example, a band gap of GaN being the nitride semiconductor is 3.4 eV, which is larger than a band gap (1.1 eV) of Si and a band gap (1.4 eV) of GaAs, and GaN has a high breakdown electric field intensity. Thus, GaN is quite promising as a material of a semiconductor device for a power supply to obtain high voltage operation and a high power.
With regard to a semiconductor device using a nitride semiconductor, many reports are made on a field effect transistor, particularly on a high electron mobility transistor (HEMT). For example, with regard to a HEMT of a GaN system (GaN-HEMT), attention is paid to an AlGaN/GaN HEMT which uses GaN as an electron transit layer and AlGaN as an electron supply layer. In the AlGaN/GaN HEMT, a deformation caused by a lattice constant difference between GaN and AlGaN occurs in AlGaN. By piezoelectric polarization caused by the above and spontaneous polarization of AlGaN, two-dimensional electron gas (2DEG) of a high density is obtained. Therefore, the AlGaN/GaN HEMT is expected as a high withstand voltage electric power device for a high efficiency switch element, an electric vehicle or the like.
Since the HEMT has an excellent high-speed performance, the HEMT is recently applied to a signal processing circuit of an optical communication system, other high-speed digital circuits and the like. The HEMT has an excellent low-noise performance in particular, and application to an amplifier in a microwave or millimeter wave band is also expected. When the amplifier is operated in the millimeter wave band, a high current gain cut-off frequency (fT) is required in order to obtain a sufficient amplifier gain. Therefore, it is necessary not only to improve mutual conductance (gm) being a parameter related to an amplification factor of a transistor but also to reduce a capacitance between a gate and a source by shortening a gate length. Further, when a module is made to be a monolithic microwave integrated circuit (MMIC) in order for miniaturization, a parasitic capacitance occurs due to an interlayer insulating film between wirings. Therefore, it becomes essential to make the interlayer insulating film have a low dielectric constant. Thus, an interlayer insulating film of a low dielectric constant (Low-k insulating film) such as benzocyclobutene (BCB) and polysilazane is applied to an MMIC wiring. Since the Low-k insulating film is low in film density and liable to absorb moisture, it is necessary to hydrophobize a surface to block penetration of water when the Low-k insulating film is used. Thus, there is developed a silica insulating film whose main skeletal structure is methyl silsesquioxane (MSQ) having a methyl group (—CH3) of high water repellency, and application to a MMIC wiring is studied.
[Patent Document 1] Japanese National Publication of International Patent Application No. 2004-532514
[Patent Document 2] Japanese Laid-open Patent Publication No. 2006-273910
[Patent Document 3] Japanese Patent No. 5071474
In addition to the aforementioned water repellency, an adhesion strength sustainable in a rear surface process or a mounting process is also required of the Low-k insulating film. In a case of the silica insulating film, a silane coupling agent is generally used in order to strengthen an adhesiveness to a base. In this method, after the silane coupling agent is made to react to a base surface, a silica insulating film is formed and a heat treatment of about 350° C. to about 400° C. is carried out. Thereby, the silane coupling agent and a hydroxyl group (—OH) contained in the silica insulating film react to each other, so that a siloxane bond (Si—O—Si) is formed in an interface by dehydration/condensation. Consequently, the adhesiveness between the silica insulating film and the base is strengthened. This method is widely used in a field of a silicon LSI.
However, in a case of a semiconductor device of which a low-temperature process is required in fabrication, for example in a case of a HEMT using a compound semiconductor, there is a possibility that a Schottky surface is reformed at a temperature of about 350° C. to about 400° C., leading to deterioration of an electric property. Therefore, it is difficult to apply a silane coupling agent in a conventional method and a new adhesion strengthening technique has been required. Further, in order to strengthen the adhesiveness by a low temperature, though a method of assisting energy necessary to reaction by ultraviolet ray is also effective, a functional group which contributes to reaction is a hydroxyl group in a case of the silane coupling agent. Therefore, an absorption wavelength of ultraviolet ray is as narrow as 200 nm or less, and a rapid effect cannot be expected. Further, there is a problem that a methyl group is desorbed when ultraviolet ray with the absorption wavelength of 200 nm or less is irradiated to a silica insulating film, leading to deterioration of water repellency.
Further, the problem is not only the adhesiveness between the protective film of SiN or the like being the base and the Low-k insulating film, but also peeling due to a low film strength in a neighborhood of a surface of the protective film. The reason why the film strength in the neighborhood of the surface in the protective film is low is that a density is low since a skeletal structure of the film is not formed due to an influence of a dangling bond in the neighborhood of the surface of the protective film. Peeling occurs in the protective film caused by such a region of a low density.